U.S. patent number 5,002,137 [Application Number 07/399,876] was granted by the patent office on 1991-03-26 for moling system.
This patent grant is currently assigned to British Gas plc. Invention is credited to Alan J. Dickinson, Peter Ward.
United States Patent |
5,002,137 |
Dickinson , et al. |
March 26, 1991 |
Moling system
Abstract
A percussive-action mole 10 is energized by compressed air
supplied through hollow rods 36 in a string 12 connected to the
mole. A hydraulic motor 18 rotates the string and mole. The mole
head 30 has a slant face 32 and a transverse permanent magnet 34.
After each new rod is added to the string, the air is stopped to
halt the mole which continues to be rotated. The field fluctuations
from the magnet are detected by a magnetometer 24 using its probe
at three positions 50, 52, 54 determiend by a triangular frame 22
placed flat on the ground. Calculations using the three readings
each representing the distance of the magnet from the respective
position on the frame enable the position and depth of the magnet
to be determined. After completion, the passage 38 can be reamed to
larger diameter to receive a gas pipe or other service.
Inventors: |
Dickinson; Alan J. (Kirby in
Ashfield, GB2), Ward; Peter (Cramlington,
GB2) |
Assignee: |
British Gas plc (London,
GB2)
|
Family
ID: |
26294344 |
Appl.
No.: |
07/399,876 |
Filed: |
August 29, 1989 |
Foreign Application Priority Data
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|
|
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Sep 2, 1988 [GB] |
|
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8820767 |
Oct 31, 1988 [GB] |
|
|
8825393 |
|
Current U.S.
Class: |
175/19; 175/45;
175/73; 324/346 |
Current CPC
Class: |
E21B
7/26 (20130101); E21B 47/024 (20130101); E21B
7/068 (20130101); E21B 47/0232 (20200501) |
Current International
Class: |
E21B
47/022 (20060101); E21B 47/024 (20060101); E21B
7/04 (20060101); E21B 47/02 (20060101); E21B
7/00 (20060101); E21B 7/06 (20060101); E21B
7/26 (20060101); E21B 007/04 (); E21B
047/024 () |
Field of
Search: |
;175/19,45,61,73,75
;324/346,326 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
246886 |
|
Nov 1987 |
|
EP |
|
247767 |
|
Dec 1987 |
|
EP |
|
266484 |
|
Nov 1987 |
|
JP |
|
1342475 |
|
Jan 1974 |
|
GB |
|
2175096 |
|
Nov 1986 |
|
GB |
|
2197078 |
|
May 1988 |
|
GB |
|
Primary Examiner: Dang; Hoang C.
Attorney, Agent or Firm: Larson and Taylor
Claims
We claim:
1. A moling system comprising a mole having a slant face at the
leading end of the mole and means for obtaining indications
representative of the plan and depth position of the mole and the
angular position of the mole about a roll axis extending lengthwise
of the mole, said mole comprising permanent magnet means having its
magnetic axis transverse to said roll axis and producing a magnetic
field extending away from the mole and penetrating into the ground
surface in a zone about the mole and said indications obtaining
means comprising magnetometer means transversable over the ground
surface following the mole and operable in response to fluctuations
of said magnetic field above the ground surface due to rotation of
said mole about said roll axis to provide said indications
representative of said angular position of the mole.
2. A system according to claim 1, said magnetometer means
comprising two magnetometer detectors one with its sensitive axis
horizontal and the other with its sensitive axis vertical, the
outputs from the detectors being passed to filter and conditioning
means and then combined in a resolver which drives a magnet coupled
to a pointer to indicate the angular position of the mole about
said roll axis.
3. A moling system according to claim 1 comprising a reference
device providing detector positions in a predetermined relationship
and detector means operable at each of said detector positions in
response to fluctuations of said magnetic filed due to rotation of
said magnet means with said mole about said roll axis to provide an
indication at each detector position representative of the distance
of said magnet means from said detector position.
4. A system according to claim 3, said detector means being
magnetometers which at each of said detector positions provides
indication of the amplitude of the magnetic field, the peak
amplitude of which is representative of said distance, and the
amplitude together with the direction of change of the amplitude of
said indication is representative of the angular position of said
mole about said roll axis.
5. A system according to claim 3 or claim 4 said reference device
providing three detector positions in a predetermined triangular
relationship.
6. A system according to claim 1 said means for obtaining
indications comprising transmitter means in the mole operable to
emit an alternating electro-magnetic field and receiver means
operable to detect said alternating field to obtain indications
representative of the plan and depth of the mole.
7. A moling system comprising a mole having a roll axis extending
lengthwise of the mole and a slant face at the leading end of the
mole, said mole further comprising permanent magnet means,
comprising a single permanent magnet having its magnetic axis
transverse to said roll axis, for producing a magnetic field
extending away from the mole, and penetrating, in use, into the
ground surface around the mole; means for rotating said mole so as
to thereby cause rotation of said permanent magnet means around
said roll axis; and measuring means for producing indications
representative of the plan and depth position of the mole and the
angular position of the mole about said roll axis, said measuring
means comprising magnetometer means, traversable over the ground
surface following the mole and responsive to fluctuations of said
magnetic field above the ground surface due to rotation of said
mole about said roll axis, for providing said indications
representative of said angular position of the mole.
8. A system according to claim 7, wherein said magnetometer means
comprises two magnetometer detectors, one with the sensitive axis
horizontal and the other with the sensitive axis thereof vertical,
for producing respective outputs; filter and conditioning means for
receiving the outputs of said detectors and for producing first and
second further outputs; and a resolver means for combining said
first and second further outputs; and for driving a magnet coupled
to a pointer to indicate the angular position of the mole about
said roll axis.
9. A moling system according to claim 7, further comprising a
reference device providing a plurality of detector positions in a
predetermined relationship and detector means, operable at each of
said detector positions in response to fluctuations of said
magnetic field due to rotation of said permanent magnet means of
said mole about said roll axis, for providing an indication at each
detector position representative of the distance of said permanent
magnet means from that detector position.
10. A system according to claim 9, wherein said detector means
comprises a plurality of magnetometers for, at each of said
detector positions, providing an indication of the amplitude of the
magnetic field, the peak amplitude of said indication being
representative of said distance, and the amplitude of said
indication together with the direction of change of the amplitude
of said indication being representative of the angular position of
said mole about said roll axis.
11. A system according to claim 9 wherein said reference device
provides three detector positions in a predetermined triangular
relationship.
12. A system according to claim 7 wherein said measuring means
comprises transmitter means in the mole for producing an
alternating electro-magnetic field and receiver means for detecting
said alternating field to obtain indications representative of the
plan and depth of the mole.
Description
The invention relates to moling systems, particularly though not
exclusively systems applicable to the installation of gas pipes or
other services in the ground.
It has been proposed in European patent application publication No.
247767 to connect a percussive mole to the leading end of a drill
pipe. The mole has a slant face at its leading end and a turning
couple acts on the mole in a plane normal to the slant face. The
drill pipe is advanced as the mole advances. The direction of
advance of the mole can thus be kept constant by rotating the drill
pipe, which rotates the mole and the slant face about the central
longitudinal axis of the mole. The direction of advance is changed
by ceasing rotation and continuing advance of the mole.
It has been proposed in GB patent application publication No.
2197078A to provide a mole with sequentially-energised coils to
generate a moving electromagnetic field which can be detected by a
remote receiver to derive an indication of the position of the mole
relative to the receiver and of roll, pitch and yaw of the
mole.
It has been proposed in GB patent application publication No.
2175096A to provide a mole with coils wound on ferromagnetic cores
to respond as receivers to a gyrating magnetic field produced by a
remote elongated ferromagnetic transmitter element rotating
relatively to a coil energised with alternating current. The
position of the mole relative to the transmitter coil and element
assembly, and the roll and pitch or yaw of the mole can be
determined by comparison of the transmitted and received
signals.
It has been proposed in U.S. patent specification No. 4621698 to
provide a mole with two coils, one aligned with the roll axis of
the mole, extending in the lengthwise direction of the mole, and
the other transverse thereto. The coils are intermittently excited
by low frequency current so as to produce corresponding magnetic
fields. The magnetic fields are detected by crossed coils
positioned in a pit excavated in the ground. The crossed coils
intersect generally on the boresite axis. Outputs from the coils
can be used to determine the angular position of the mole about the
roll axis and the angular position of the roll axis in relation to
the horizontal and vertical direction.
A moling system according to the present invention comprises a mole
having a slant face at the leading end of the mole and means for
obtaining indications representative of the plan and depth position
of the mole and the angular position of the mole about a roll axis
extending lengthwise of the mole, said mole comprising magnet means
having its magnetic axis transverse to said roll axis and producing
a magnetic field extending away from the mole and said means
comprising magnetometer means operable in response to fluctuations
of said magnetic field due to rotation of said mole about said roll
axis to provide said indications representative of said angular
position of the mole.
According to one preferred form of system, said magnetometer means
comprise two magnetometer detectors one with its sensitive axis
horizontal and the other with its sensitive axis vertical, the
outputs from the detectors being passed to filter and conditioning
means and then combined in a resolver which drives a magnet coupled
to a pointer indicating the angular position of the mole about said
roll axis.
In another preferred form of the method, the plan position and
depth of the mole are determined using a reference device providing
detector positions in a predetermined relationship and detector
means operable at each of said detector positions in response to
fluctuations of said magnetic field due to rotation of said magnet
means with said mole about said roll axis to provide an indication
at each detector position representative of the distance of said
magnet means from said detector position.
Preferably, said detector means are magnetometers which at each of
said detector positions provides indications of the amplitude of
the magnetic field, the peak amplitude of which is representative
of said distance, and the amplitude of the fluctuation and the
direction of the change of the amplitude at any time are
representative of the angular position of the mole about its roll
axis.
Preferably, said means for obtaining indications comprise
transmitter means in the mole operable to emit an alternating
electro-magnetic field and receiver means operable to detect said
alternating field to obtain indications representative of the plan
and depth of the mole.
Embodiments of a moling system and preferred ways of using it to
perform methods of moling will now be described by way of example
with reference to the accompanying drawings, in which:
FIG. 1 is a diagrammatic longitudinal, vertical section through the
ground showing the system in use;
FIG. 2 is a diagrammatic plan showing a triangular reference device
position about ground over a mole below ground;
FIGS. 3 and 4 are diagrams showing a triangle made up of one side
of the reference device shown in FIG. 2 and two sides having
lengths representing the distances between the magnet means in the
mole and two detector positions one at each end of the side of the
reference device;
FIG. 5 is a diagrammatic vertical section through part of a second
embodiment of system;
FIG. 6 is a section on the line VI--VI in FIG. 5;
FIG. 7 to 10 show diagrammatically the variation in the output of
the magnetometer with roll angle;
FIG. 11(a) is a diagrammatic vertical section through part of a
third embodiment of system and FIG. 11(b) shows diagrammatically
the direction of rotation of the mole head;
FIG. 12 shows magnetometer outputs for different roll angles for
the system of FIG. 11; and
FIG. 13 shows the magnetometer detectors of the system shown in
FIGS. 11(a) and 11(b).
The moling system shown in FIG. 1 consists of the following
principal components: a pneumatically operable percussive mole 10;
a string 12 of hollow drill rods connected end-to-end; a launching
frame 14; a hydraulic power pack 16 supplying a hydraulic motor 18
on the frame 14 arranged to rotate the string 12; a source 20 of
compressed air to power the mole 10; a triangular reference device
22 normally positioned flat on the ground but shown vertical for
clarity; three magnetometer detectors 50,52,54 one at each corner
of the reference device; and signal conditioning and display device
24.
FIG. 1 includes an enlarged detail showing the head 30 of the mole
10. The head 30 is of stainless steel and has a slant face 32. The
head 30 has a transverse bore containing magnetic means in the form
of a bar magnet 34; alternatively the magnet means are two thin
section, rare earth magnets mounted in recesses on either side of
the mole head; alternatively the magnet means is an
electromagnet.
The string 12 is shown containing three rods 36 and the leading rod
is connected to the trailing end of the mole 10. Typically, each
rod 36 is 1.5 meters long.
The system is, for example, used to form a pilot passage 38,
typically of 50 millimeters diameter, which would subsequently be
reamed out to a larger diameter to receive a gas distribution pipe,
for example of 125 mm outside diameter.
The mole 10 displaces earth as it advances under the precussive
action of an internal hammer driven by pneumatic pressure. The
slant face 32 on the head 30 of the mole gives rise to a transverse
reaction from the earth which causes the path of the mole to curve
in the direction opposite to that in which the face is directed.
With the mole positioned as shown in FIG. 1 the path of the mole
would curve downwardly, assuming the mole did not rotate about its
roll axis 40 which extends in the lengthwise direction of the mole.
In order to maintain the mole on a generally straight path the
hydraulic motor 18 is operated to rotate the string 12 as the mole
advances. The mole's path is then a corkscrew-shaped path of very
small radius and approximates to a straight path. The pilot passage
38 shown in FIG. 1 is formed initially as the mole 10 is launched
from the frame 14 into the ground at a small angle to the
horizontal. Then, the mole's path is made to curve towards
horizontal by setting the mole's angular position about its roll
axis so that the slant face 32 faces downwardly.
As the mole progresses, it is necessary to monitor the mole's
position beneath the ground in both the horizontal and the vertical
planes. It is also necessary to monitor the mole's angular position
about its roll axis 40. Such monitoring is performed using the
reference device 22 and signal conditioning and display means
24.
The reference device 22 is preferably for example a frame in the
form of an isosceles triangle having two equal sides, which
provides three detector, positions 50,52,54 at which magnetometer
detectors are positioned. The detectors are connected by a lead 56
to the signal conditioning and display unit 24.
The signal conditioning and display unit has a meter with a pointer
which responds to the fluctuating magnetic field, and a means of
capturing and displaying on a digital meter the value of the peak
amplitude signal from each of the three detectors.
When the mole rotates about its roll axis, typically at between 20
and 60 revolutions per minute for example, the rotation of the
magnet 34 causes fluctuation of the magnetic field about
ground.
The response of the magnetometer means to that fluctuation is
super-imposed on the effect of the earth's field. The needle on the
magnetometer unit 24 oscillates about zero, owing to the earth's
and other stray magnetic fields being compensated for either by
electronic means (e.g. AC coupling) or by magnetic means. The
peak-to-peak reading from each sensor is a measure of the distance
of the magnetometer sensor from the magnet 34.
For each revolution of the mole about its roll axis 40, the needle
travels from full left to full right and back to full left
deflection. The direction of travel of the needle as well as its
position can thus indicate the angular sense of rotation of the
mole and can be used to set the angular position of the slant face
32 about the roll axis 40.
In monitoring the progress of the mole, the magnetometer means are
used to obtain, for each of successive locations of the mole 10, a
group of three peak amplitude readings. Each such location is
reached by the mole after the advance for a given rod 36 has been
completed. In other words, those locations occur every 1.5 meters.
At each location, the forward progression of the mole is
temporarily halted but the string 12 and the mole are rotated by
the motor 18. The frame 22 is placed flat on the ground over the
approximately known path of the mole with the apex of the triangle
(i.e. the detection position 50) pointing in the approximate
direction of advance of the mole.
For each location of the mole, the group of three readings is used
to calculate the depth, the longitudinal position and plan position
of the magnet 34 as will be explained next, with reference to FIGS.
2, 3 and 4.
In FIG. 2, the three corners A,B,C of the triangular frame
correspond to the detector positions 50,52,54 respectively. The
point G is in the plane of the frame and vertically above the
magnet position M. The triangular frame is constructed in the form
of an isosceles triangle with the equal sides extending from the
apex that points in the direction of moling. For the system
described here, the lengths of the equal sides are chosen so that
the length of the base is 0.5 m and the distance from the base to
the apex is 0.5 m. Whilst the calculations which follow will be
valid for any isosceles triangle, the accuracy of the calculation
of mole position will depend on the detector spacing and the depth
of the mole. The dimensions of the triangular frame are a
compromise between location accuracy and a convenient size for use
of the detector frame.
In FIGS. 2 and 3, position D is the mid-point of the line BC.
In FIG. 3, M is the position of the mole head and a perpendicular
from the mole (M) to the base line (BC) intersects at point X.
In FIG. 4, the line AD is the centre line of the detector frame and
this line should be aligned with the intended path of the mole
(i.e. the target line). Position Y is the intersection between the
centre line of the frame (AD) and the perpendicular constructed
from this line to the mole head.
At each location, the peak output from the three magnetometer
detectors at positions A, B and C is a function of the distances of
those positions from the magnet at the point M. In other words the
distances AM, BM and CM can be determined by calculation from the
detector outputs using equation 1:
where S is the distance of the magnet from the detector, k.sub.l,
k.sub.2, k.sub.3 are constants, V is the peak output signal from
the detector and P is the out-of-plane angle, i.e., the angle
between the plane of rotation of the magnet and the line joining
the magnet to the detector.
It can be shown that for detectors at positions B and C, the
out-of-plane angle P is given by EQU 2:
where GX.sup.2 =(BM.sup.2 -BX.sup.2 -GM.sup.2) and GM is the
vertical depth of the magnet.
For the detector at position A, the out-of-plane angle is given by
equation 3:
The value of the distance S from the magnet to a detector
(corresponding to the distances AM, BM, CM) is calculated using as
a first approximation an out-of-plane angle P=0. From these first
approximations a first estimate of the location of the magnet can
be calculated in terms of XM, YM and GM. From the first estimate of
the position of the magnet, the out-of-plane angle can be derived
approximately using either equations 2 or 3. The magnet position
can then be recalculated and a better estimate of angle P obtained.
Three itterations give a sufficiently accurate estimate of the
magnet position.
The calculation of depth plan and longitudinal position is split
into three parts. The first part calculates the sideways plan
position (i.e. the X value) using the equation 4:
where BC is known from the dimensions of the detector frame and BM
and CM are calculated from Equation 1.
The second part calculates the longitudinal position (i.e. the Y
value).
To determine the Y position, the magnetometer outputs from the
detectors at positions B and C are combined to establish an
estimate of the signal that would be seen by a detector at the mid
point position D on the baseline, and then the estimated signal is
used with the signal from the detector at the apex A to calculate
the Y position.
To generate the signal from the imaginary sensor at D, first the
distance from X to the magnet (XM) is calculated from equation
5:
then the distance from D to the magnet is calculated using equation
6:
then using the distance DM in equation 1, an estimate is made of
the peak output voltage which would be produced by a detector at D.
Finally, the distance DY (i.e. the Y position) is calculated from
equation 7:
The third part of the process calculates the depth of the mole
below the X,Y coordinate point (G) by calculating the distance from
Y to the magnet YH using
and then calculating the vertical depth (GM) from
The various calculations are conveniently and quickly performed by
a microcomputer using a relatively simple programme so that the
position and depth of the mole can readily be made in the field as
moling progresses without unduly delaying the moling procedure.
Alternatively the outputs from the three detectors can be passed
directly into the microcomputer, increasing the speed of the system
and reducing the chance of operator error.
FIG. 1 shows a small excavation 60 which is intended to allow, for
example, a connection to be made into the gas pipe or other service
which is installed either in the passage 38 or in a passage of
larger diameter formed by reaming out the passage 38. The part of
the passage 38 leading from the surface of the ground to the
excavation 60 would not normally be required to receive a gas pipe
or other service and functions purely as a pilot entry passage for
the rod string 12 during moling.
FIGS. 5 and 6 show an alternative system in which the following
features are shown:
Detector means 150, preferably a fluxgate magnetometer e.g. type
LPM2 available from Thorn EMI Limited; further detector means 152,
preferably a receiver unit type RD300 available from Radiodetection
Limited having two solenoid coils 154,156 one above the other; the
surface of the ground is shown at 158; the head 130 of the mole
110; the slant face 132 on the head 130 and the transverse bore
containing the permanent magnet 134. The magnet 134 is preferably
an Alnico alloy type available from Buck and Hickman. It is 30
millimeters long and 10 mm in diameter. it gives a peak field
strength of 10 micro-tesla at 0.3 meter from the magnet. The
magnetic axis is transverse to the roll axis 140 of the mole
110.
Alternatively rare earth type magnets can be used as in the
configuration shown in FIG. 1. These give a peak field strength of
100 micro-tesla at 0.3 m from the magnet.
If the mole rotates at 20 revolutions per minute, the field varies
effectively at the ground surface at 0.3 H.sub.z.
The head 130 consists of two parts: the leading part of toughened
steel providing the slant face 132 and a non-magnetic stainless
steel carrier 162 for further detector means in the form of a sonde
166. The sonde 166 is preferably a re-packaged version of a small
sonde available from Radiodetection Limited. The sonde 166 is
located in a transverse slot in the carrier 162 and retained by a
sleeve 167. The sonde 166 typically measures 40 mm.times.40
mm.times.13 mm and is supported by a rubber mounting to isolate it
from impact forces. The sonde 166 contains integrally encapsulated
rechargeable batteries and transmits an electromagnetic field at a
preferred frequency of 33 kiloherz, though a range of 8-125
kH.sub.2 is available. The transmitter is designed so that the
field is uniform about the roll axis of the mole.
The magnetometer 150 and the receiver 152 preferably form a single
transportable unit indicated at 169. The output from the coils 154,
156 is amplified, filtered to reduce interference, rectified and
displayed on a moving coil meter. The detection range is better
than 1.5 meter.
The sensitive axis 170 of the magnetometer 150 is arranged
vertically. Peak positive response is obtained when the north pole
of the magnet 134 is pointing vertically towards the magnetometer
150 and zero response is obtained when the axis of the magnet 134
is horizontal. FIGS. 7 to 10 show the meter outputs of the
magnetometer 150 as the mole rotates through 360.degree. about its
roll axis. Starting at FIG. 7 with the magnet axis vertical and the
north pole uppermost, the meter output is a positive, clockwise
maximum corresponding to a starting angular position of 0.degree..
FIG. 8 shows the meter output at midscale i.e. zero corresponding
to 90.degree. rotation. FIG. 9 shows meter output at negative,
anti-clockwise maximum corresponding to 180.degree. rotation. FIG.
10 shows the meter at mid-scale, i.e. zero corresponding to
270.degree. rotation of the mole.
The output from the magnetometer is amplified with an AC coupled
amplifier with a low frequency cut-off at 0.03 H.sub.z. The AC
coupling removes the large offset caused by the vertical component
of the earth's magnetic field. The amplifier has adjustable gain
and the output is fed to the centre-zero moving coil meter which
gives the scale indications shown in FIGS. 7 to 10.
As the mole rotates the meter output fluctuates as already
explained, the needle oscillating about the centre zero. The
magnitude of the peak response depends on the distance of the
magnet 134 from the magnetometer and the gain setting of the
amplifier. The gain setting is adjusted, once the oscillations have
begun, until the meter needle travels from the full anti-clockwise
position to the full clockwise position. By noting the position and
direction of travel of the needle, the instantaneous angular
position of the slant face 132 can be determined. The rotation of
the mole can be halted with the slant face 132 in a predetermined
orientation so that subsequent advance of the mole without rotation
effects a desired change in the direction of advance.
The plan position of the mole is determined by sweeping the
transportable unit across the ground. The field strength of the
electromagnetic field emitted by the sonde 166 varies with distance
so when a maximum output is observed from the receiver 152, the
receiver is known to be above the mole. The two coils 154, 156
enable the field strength and the field gradient to be measured
which enables the depth of the mole to be determined.
The determination of the plan position depth and angular position
of the mole is carried out at successive intervals, preferably
after each new rod (corresponding rods 36 of FIG. 1) is added.
During the determination the air supply to the mole is discontinued
so that the mole is not advancing. However, the motor
(corresponding to motor 18 of FIG. 1) continues to run so that the
mole is still rotating about its roll axis 140.
Once the determination has been completed, the mole either
continues as before or, if a correction is required in its
direction of advance, the mole is advanced without rotation, the
mole's angular position about the roll axis 140 having been set so
that the slant face is oriented to produce a desired correction to
the line of advance. The amount of correction achieved is checked
at the next determination of position and if necessary, further
advance without rotation is effected, and so on.
Another embodiment of system is shown in FIGS. 11(a), 11(b), 12 and
13 in which two magnetometer detectors replace the single
magnetometer detector shown in FIG. 5. The receiver unit 52 would,
of course, still be used.
This embodiment can also be used in the system described with
reference to FIGS. 1 to 4 by using four magnetometer detectors
there being two magnetometer detectors at one of the corners of the
triangular frame. The two magnetometer detectors are placed close
together directly above the magnet position. The two detectors are
arranged with the sensitive axis of one in a vertical direction and
the sensitive axis of the other in a horizontal direction in the
plane of rotation of the magnet.
As the mole head (and thus the magnet) rotates the signal from both
detectors will be sinusoidal but because of the different
orientation of the two detectors there will be a 90.degree. phase
difference between the outputs so that one detector output will
describe a sine function and the other detector output will
describe a cosine function.
The signals from the two detectors also contain a D.C. component
resulting from the effect of the earth's magnetic field and other
magnetised objects in the vicinity. The signals are therefore
passed to a signal conditioning unit which filters the D.C.
component leaving just the sinusoidal components of the two
signals. The signals are then passed to a display device which
consists of a D.C. Resolver which drives a pointer round a circular
scale.
FIGS. 11(a) and 11(b) show the arrangement of the detection in
relation to the mole head. The view of the mole head in FIG. 11(a)
is along the longitudinal axis of the mole with the magnetic axis
transverse. As the mole head rotates, the magnet generates a
varying magnetic field at the ground surface. If the speed of
rotation is reasonably constant then the magnetic field at the
ground surface varies sinusoidally.
Detector B is arranged with its sensitive axis in a vertical
direction so that as the magnet rotates, the output from the
detector has a peak positive value when the north pole of the
magnet points towards the sensor and a peak negative value when the
south pole of the magnet points towards the sensor.
In addition to this varying field the detector will also respond to
the vertical component of the earth's magnetic field. The resultant
output from the detector is shown in FIG. 12.
Detector A is arranged with its sensitive axis in a horizontal
direction in the plane of rotation of the magnet. As the head
rotates the output from this detector has a peak positive value
when the magnet is horizontal with its north pole pointing to the
left, and a peak negative value when the south pole points to the
left. In addition to the varying field the detector will also
respond to the horizontal component of the earth's field. The
resultant output of detector A is shown in FIG. 12.
As shown in FIG. 13, the output from detectors A and B are passed
to two signal conditioning units which filter out the DC component
and then amplify the signal to the correct level to drive the DC
resolver.
The DC resolver comprises two coils, A and B arranged at right
angles with a magnet pivoted about its centre. Coil A is driven by
the cosine signal from detector A and coil B is driven by the sine
signal from detector B. Each coil generates a magnetic field
proportional to its excitation current and the resultant field is
the algebraic sum of the fields generated by A and B.
If the peak amplitude of the fields generated by coils A and B are
the same then the resultant is a constant amplitude magnetic vector
rotating at a velocity determined by the period of the excitation
signals. The rotating magnetic vector thus has the effect of
causing the pivoted magnet to rotate and mimic the rotation of the
magnet in the head of the mole. A pointer is fixed to the magnet in
the Resolver and the circular scale indicates the angular position
of the mole head. Thus, by stopping rotation when the head is in a
desired position the mole's course can be corrected as
required.
The advantages of this technique are that:
1. The pointer gives a clear visual indication of the orientation
of the mole head.
2. The operation of the DC resolver depends on the relative
amplitudes of the signals applied to coils A and B which are
affected equally by changes in depth. There is therefore less need
for the operator to accurately adjust the signal amplitude in order
to get an accurate indication of roll angle.
* * * * *